With the recent popularization of computers, an inkjet printer is widely used for printing letters or an image on paper, film, cloth or the like not only at offices but also at homes.
The inkjet recording method includes a system of jetting out a liquid droplet by applying a pressure using a piezoelectric element, a system of jetting out a liquid droplet by, generating a bubble in an ink under heat, a system of using an ultrasonic wave, and a system of ejecting a liquid droplet by drawing with an electrostatic force. The ink composition used for such inkjet recording includes an aqueous ink, an oily ink and a solid (fusion-type) ink. Among these inks, the aqueous ink becomes mainstream in view of production, handleability, odor, safety and the like.
The coloring agent used in such an ink for inkjet recording method is required to have high solubility in a solvent, enable high-density recording, provide good color hue, exhibit excellent fastness to light, heat, air, water and chemicals, ensure good fixing property and less bleeding on an image-receiving material, provide an ink having excellent storability, show no toxicity, have high purity, and be available at a low cost. However, it is very difficult to find out a coloring agent satisfying these requirements in a high level.
Various dyes and pigments for inkjet recording method have been already proposed and actually used, however, a coloring agent satisfying all requirements is not yet found out at present. Conventionally well-known dyes and pigments having a Colour Index (C.I.) number can hardly satisfy both color hue and fastness required of the ink for inkjet recording method. Heretofore, various studies have been made on dyes having good color hue and fastness with an attempt to develop a dye excellent as the coloring agent for inkjet recording method. However, in a compound called a water-soluble dye, a water-soluble group is necessarily substituted. When the number of water-soluble groups is increased so as to improve the stability of ink, it has been found to cause a problem in that the formed image readily bleeds under high-humidity conditions.
The present inventors have found that a betaine compound is effective for solving the bleeding phenomenon.
However, it is revealed that when a betaine compound is used in an ink set comprising dark and light inks having the same color hue, a sufficiently high effect of restraining the bleeding cannot be obtained unless the concentration of betaine compound in each ink is controlled.